The suit alleges, among other things, that the RIAA used low and downright illegal tactics to pry information out of people. It also claims that the RIAA and MediaSentry both know they are using illegal tactics and have flawed investigations:
MediaSentry and the RIAA know that their investigations are illegal and flawed. MediaSentry is not licensed or registered to conduct private investigation of private US citizens. Moreover, in a March 2004 sworn deposition MediaSentry's then president admitted to various serious flaws in the investigative scheme which all Defendants know result in misidentification of individuals.
On top of that, the suit also claims the company as a whole uses fraud, trespasses and invades the privacy of individuals all to forward their goal of maintaining a music monopoly. A lot of people have come to that conclusion without being sued or suing the RIAA, though clearly we are seeing more and more people standing up not only for themselves, but for any potential future targets of the RIAA. You can read more about the suit from the RIVP blog.